The immersion process, the flow-coating process or a pour-coating process may be utilized to paint hollow objects of this type. There are certain advantages and disadvantages inherent to each of these processes. In the immersion process the largest possible submersion vats are used, depending on the size of the parts to be coated and the required throughput rate. If hollow objects are to be coated in the immersion process, a problem is encountered in that the hollow objects have to be filled completely with the paint, without air bubbles, when immersed and that for this reason they must enter the submersion bath with the opening upward whereas after treatment, i.e. after surfacing from the submersion bath, they have to be emptied again, i.e. the opening must be turned downward. This necessitates complex engineering for the conveyor devices for the hollow objects. A process of this type is, for instance, described in European Patent Application 0 118 756.
In the electrophoretic flow-coating process a coating is applied by simply flow-coating or pour-coating an object with the assistance of an electrophoretic paint. Here it is to be noted that the velocity of the paint as it flows over the object is subject to narrow limits so as not to interfere with the formation of the coating. Furthermore, it is possible in this fashion to apply a coating to only one surface, in hollow objects only to the outer surface.
To reach the inside of hollow objects as well, it has been further suggested in German published examined patent application 26 33 179 that a nozzle tube be inserted into the hollow objects which are passed along and above a catch basin and to cause the liquid electrophoretic coating material to flow through the nozzle pipe and onto the inside surface of the hollow object while this hollow object is being rotated around its axis. A particularly complicated mechanism is required in this process since the hollow objects must be held in a suitable holder and rotated around their own axes while in addition the nozzle pipes have to be inserted axially into the hollow objects and then retracted. Furthermore, in the flow-coating and nozzle pipe processes, both requiring a catch basin, considerable foam formation take place which must be counteracted either by mechanical means which require a considerable amount of space or by chemical means which interfere with the properties of the electrolytic liquid.